In the prior art there is described a method of measuring the degree of oxygen saturation of blood using what is commonly known as the optical pulse oximetry technology. References to that technology may be found in U.S. Pat. Nos. 4,167,331, 4,938,218, in the brochure "Fetal Oxygen Physiology" sponsored by NELLCOR LTD., and there are others. In accordance with this method, a blood perfused tissue is illuminated and light absorption by the tissue is determined by a suitable light sensor. Pulsatile changes in the value of absorption which are caused by cardiovascular activity of the blood are then used to determine the characteristic of interest, i.e. the degree of blood oxygen saturation.
The value of oxygen saturation (SaO.sub.2) in arterial blood is defined by the following known equation: ##EQU1## where [HbO.sub.2 ] is the concentration of oxygenated hemoglobin concentration in a unit of blood volume and [Hb] is the concentration of reduced hemoglobin.
In commonly used methods of pulse oximetry a tissue under investigation is illuminated by light having at least two components of different wavelengths, and the measurements are based upon the following two physical phenomena:
(a) the light absorbance of oxygenated hemoglobin is different from that of reduced hemoglobin, at each of the two wavelengths;
(b) the light absorbance of the blood perfused tissue at each wavelength has a pulsatile component, which results from the fluctuating volume of arterial blood passing across the tissue between the light source and the sensor.
It is therefore assumed, that the pulsatile absorbance component of a tissue layer located between the light source and the sensor characterizes the degree of oxygen saturation of arterial blood.
Various types of sensors designed for effecting measurements in the performance of optical pulse oximetry are known in the art, and among the known optical sensors those dedicated to measuring the degree of oxygen saturation of fetal arterial blood constitute a particular group of such devices.
Basically, the prior art discloses two types of optical sensors which are associated with and serve for two modes of performing optical blood oximetry: transmission pulse oximetry in which so-called transmissive sensors are used and reflection pulse oximetry in which so-called reflectance or transflectance sensors are used. In transmission pulse oximetry one measures light passing across a blood perfused tissue such as a finger, an ear or the like by placing a light emitter and the detection of a transmissive sensor at two opposite sides of the tissue under examination, as described for example in U.S. Pat. No. 4,938,213. In reflection oximetry, on the other hand, reflectance or transflectance sensors can be used which comprise both light emitters and light detectors which are accordingly placed on one and the same side of the tissue under examination, as described, for example, in U.S. Pat. Nos. 5,228,440, 5,247,932, 5,099,842 and in WO 90/01293. Reference to the two types of sensors can also be found, for example, in U.S. Pat. No. 5,247,932 and in "Fetal Oxygen Saturation Monitoring" sponsored by NELLCOR.
Both the transmission and the reflection modes of operation have specific limitations of applicability and their accuracy in general, and in specific applications in particular is not satisfactory. This, for example, the transmission technology can be successfully applied only in cases where the tissue to be investigated forms a distinctive protrusion which makes it possible to apply a light emitter and a light sensor at opposite surfaces.
It is thus evident that the reflection technology is the one most commonly resorted to, notably in fetal oximetry. Unfortunately, however, accuracy of the conventional reflection technology is rather low in comparison with that of the transmission one, because the degree of diffusion of the emitted light in the tissue is unknown, which means that the nature of the functional interdependence between a light signal received by the sensor and the degree of blood oxygen saturation is also unknown. Another disadvantage of the known reflection technology is a partial shunting of the emitted light on the surface of the tissue between the source and the sensor, and a specular reflection created by the superficial layer of the tissue.
U.S. Pat. No. 5,009,842 describe a sensor with means for overcoming the problem of shunting of the emitted light on the outer surface of the tissue between the light source and the detector. U.K. Patent Application No. 2 269 012 proposes to select and separate light signals resulting from light reflection by a superficial layer of a blood perfused tissue such as skin or hair, essentially by choosing a particular distance between the locations of emitting and detecting optical fibers on the contacted surface of the tissue under examination.
Fetal oximeters usually comprise applicators which generally include a plate with at least one substantially point-like light source and at least one substantially point-like light detector suitably spaced from the light source(s). One drawback of such applicators is that if the applicator is applied to a non-uniform section of the skin, such as a hairy portion or a birthmark, the light signal received by the detector(s) will be distorted. Even in relatively large size oximetry, e.g. of the kind described in U.S. Pat. No. 5,099,842 the light sources and detectors are still point-like and accordingly it is practically unadvoidable for an operator to apply it to a wrong portion of the skin of a fetus.
It is important to recall that the basic assumption underlying the theory of transmission and reflection oximetry is, that optical paths of light rays with different wavelengths emitted in the tissue by different light sources, are substantially equal. However, in actual fact the length of such an optical path depends on the light scattering coefficient which, in its turn, is a function of the wavelength. Accordingly, when the wavelengths of the light sensors chosen for oximetry measurements and with them the light scattering coefficients significantly differ from each other, the basic assumption of substantial equivalence of optical paths is violated.
In cases where two or more point-like light sources are used, problems may arise due to the fact that the skin surface, blood vessels and other parts of biological media, are not structured and distributed homogeneously. Thus, if one point-like light source emitting at a given wavelength is applied to any site of a non-uniform skin, while the other light source emitting at a different wavelength is attached to a topographically adjacent but optically different site, then in consequence of different light scattering and absorption at the two distinct wavelengths, which occurs from the very beginning, the optical paths of the light emitted by the two sources cannot be equal. The total amount of optical energy acquired by a detector can be approximated as being the sum of the amounts of energy portions carried by the propagating rays reaching the detector. As the optical paths of these rays are wavelength-dependent and since each part of that energy travels to the detector through a different optical path, the total attenuation of light components with different wavelengths can significantly differ from each other, with the consequence of the occurrence of a random error in the evaluation of oxygen blood saturation.
Another drawback of known sensors for blood oximetry is that they utilize LEDs as light sources for illuminating a tissue with light having two wavelength components. The LED light sources are either installed in the probe itself such as, for example, in U.S. Pat. No. 4,938,218 or linked to the probes via optical fibers such as, for example, in U.S. Pat. No. 5,099,842, GB-A-2 269 012, WO 91/18549 and WO 90/01293. such light sources may provide, for example, a pair of wavelengths of 700 nm and 800 nm which are suitable for the purposes of blood oximetry. However, although it is well known that the accuracy of oximetric measurements increases the closer the two wavelengths are to each other, nevertheless within the wavelength range required for oximetry LEDs are incapable of providing two wavelengths closer to each other than 100 nm.